H2O and F contents in Mt. Hood magmas recorded by plagioclase phenocrysts

Characterizing volatile contents of andesite magmas is crucial to understanding the history of crustal magma chambers and the eruption dynamics of arc stratovolcanoes. The H₂O and F in plagioclase (Pl) phenocrysts may provide an important record of volatile contents in their host magmas. Here we report H₂O and F concentrations in Pl phenocrysts from recent eruptions at Mt. Hood in the Oregon Cascades. Paired with Pl/melt partition coefficients [1], these can be used to calculate the H₂O and F contents of the melt.

H₂O and F contents of Pl in pumice from the three most recent eruptive units (Old Maid - 0.2 ka, Timberline - 1.5 ka, and Polallie - 13-20 ka) were measured with secondary ion mass spectrometry (SIMS) following techniques in [2]. Profiles across single crystals were taken to assess the possibility of degassing. The measurements can be compared to melt inclusion (MI) data from the same units [3].

Timberline and Polallie Pl have varying H₂O diffusion profiles with some minimally degassed phenocrysts containing up to 100±2 μg/g and 114±2 μg/g, respectively. These likely retain H₂O concentrations approaching equilibrium with the melt. Using D_H2O = 0.002 between Pl and melt ([1], with revised values after [2]), the corresponding H₂O contents in the Timberline and Polallie melts are 5.0 and 5.7 wt.%, respectively, similar to the maximum MI H₂O (5.4 and 5.1 wt.%) [3]. Old Maid phenocrysts have diffusion profiles consistent with significant volatile loss and maximum H₂O of 42±2 μg/g. The corresponding H₂O in the melt is 2.2 wt.%, much less than the 5.4 wt.% H₂O recorded in MI [3].

F contents vary less than H₂O within each phenocryst and between different phenocrysts. Pl from all 3 eruptive units each contain maxima of 4 μg/g F. for andesite is not known, but preliminary experiments on basalt give D_F = 0.007 between Pl and melt. This corresponds to maxima of 570 μg/g F in the melt, in agreement with 500 μg/g F in MI in [3]. Favorable comparison of Pl H₂O and F contents and associated MI at Mt. Hood suggests Pl may provide a detailed record of H₂O and F melt contents. Further experimental work to constrain and would enhance the accuracy of calculated H₂O and F in the melt.

[1] Johnson, Goldschmidt, A743 (2005)

[2] Mosenfelder et al., Am Min 100, 1209-1221 (2015)

[3] Koleszar et al., JVGR 219-220, 1-14 (2012)